The subject matter disclosed herein relates to combustors for a high-temperature gas turbine engine, and more particularly, to combustor housings for use with fuel gas having a low BTU content.
Uncertainties in the cost and availability of petroleum-based fuels and natural gas, coupled with the desire to use all available fuel sources have resulted in commercial interest in the use of very low heating value gas fuels to power gas turbines. Various designs have been proposed for the use of low heating value gases, such as those having heating contents as low as 2500 BTU/lbm and have generally consisted of adaptations of existing combustor designs.
One example is U.S. Pat. No. 4,498,288 that describes a combustor design whereby a portion of a low-BTU gas fuel is injected through a traditional style fuel nozzle into a primary burning zone and the balance of the low-BTU gas fuel is injected through a secondary pipe into a main burning zone. This combustor design does not consider or accommodate the need for large flow areas upstream of combustion.
Another example, EP0310327A3 describes a combustor that is similar to that described in U.S. Pat. No. 4,498,288 above in that the balance of the low-BTU gas fuel is injected through a secondary passage. However, in this case, both the primary and secondary injection passages discharge into a single combustion zone. This combustor design also does not consider or accommodate the need for large flow areas upstream of combustion.
Still another example, U.S. Pat. No. 6,201,029 contains another approach to burning low-BTU content gas fuels that again involves downstream injection of a portion of the low-BTU content fuel gas. This combustor design also does not consider or accommodate the need for large flow areas upstream of combustion.
Still another example, US2007/0275337 describes a combustor in which a helical air swirler is modified to include fuel injection into the swirling air passages, and indicates that such injection is well-suited to the combustion of low-BTU synthesis gas and that such a burner can operate in pre-mixed or diffusion mode and can handle low or high levels of fuel heating value with different fuel injection circuits. The radial air passages are large relative to the conventional fuel passages, such that area is available for injection of low BTU gas. This combustor design also does not consider or accommodate the need for large flow areas upstream of combustion.
As the calorific value (BTU or energy content, or Lower Heating Value (LHV)) of gas fuels is reduced, the required flow rate increases. This leads to an increased pressure loss through passageways originally designed for fuels with higher energy content. This loss of pressure comes at a great price to the turbine cycle efficiency if the fuel compressor is driven by the gas turbine. This problem is exacerbated in fuel gases having a very low calorific value, such as those having a calorific value less than 2500 BTU/lbm. It is even further exacerbated as the number of separate inlets used for the very low energy content gas are increased to provide the amount of such gas necessary for combustion. Gas velocities through fuel flow passages may also be high, leading to increased heat transfer from the metal walls to the fuel gas, or vice versa. This can cause local thermal gradients within the combustor leading to increased cyclic thermal stresses and the possibility for degradation or failure of the various combustor components. The design, development, machining and other manufacturing processes employed to create multiple fuel flow passages into the combustor or combustion chamber adds complexity and cost to systems using multiple passages.
Therefore, it is desirable to reduce pressure losses associated with the use of very low content fuels so as to improve the system efficiency. It is also desirable to reduce gas velocity to reduce thermal gradients and associated thermal stresses, particularly where the fuel gas exits the combustor through the nozzle, since this is the high-temperature portion of the combustor. It is also desirable to simplify the combustor design, particularly as it relates to the incorporation of multiple fuel lines into the combustor to lower the complexity and cost of the combustor.